1. Field of the Invention
The present invention relates to an extrusion molding apparatus for molding a ceramic molded product such as a honeycombed structure.
2. Description of the Related Art
A ceramic molded product 8 having a honeycombed structure, which is configured so that partitioning walls 81 for partitioning a multiplicity of cells 88 formed through in an axial direction are arranged in the form of a honeycomb as shown in FIG. 16, for example, is used as a catalyst carrier of an exhaust gas purification system of automobiles. This ceramic molded product 8 is generally fabricated by extrusion molding of a mixed ceramic material 80 (FIG. 17).
An extrusion molding apparatus for the ceramic molded product 8, as shown in FIG. 17 as an example, comprises a shaping die 91 for molding a honeycombed structure, and two screw extruders 98 in two stages for mixing and extruding the ceramic material 80 continuously.
The two screw extruders 98 may be replaced with either three or more screw extruders or a single screw extruder with equal effect.
As shown in FIG. 17, the ceramic molded product 8 is molded using an extrusion molding apparatus 9 having the two screw extruders 98 in such a manner that the ceramic material 80, supplied to the upper screw extruder 98 is mixed and advanced by the upper screw extruder 98 and, through a filter 94, is supplied further to the lower screw extruder 98. The lower screw extruder 98 mixes and advances the supplied ceramic material 80, and extrudes it from a shaping die 91 through a filter 93 and a resistance pipe 92. In this way, the honeycombed ceramic molded product 8 is produced.
At least the lower screw extruder 98 has built therein an extruding screw 99 for extruding the ceramic material 80 toward the shaping die 91. This extruding screw 99 is so configured as to mix the ceramic material 80 while at the same time pressing the ceramic material 80 toward the shaping die 91 to thereby extrusion-mold the ceramic material 80 (See Japanese Unexamined Patent Publication No. 2000-238022, for example).
As shown in FIG. 18, however, the ceramic molded product 8 extrusion-molded by the screw extruders 98 develops an uneven portion 89 high in density and low in fluidity of the ceramic material 80 at a position corresponding to the axial center of the extruding screw 99. This uneven portion 89 represents a portion where the ceramic material 80 pressed along the outer surface of the extruding screw 99 and increased in density is concentrated. In the case of a white ceramic material, this uneven portion 89 is visible as a black portion as compared with the surrounding portions.
A conventional extrusion molding apparatus, proposed to eliminate this uneven portion, as shown in FIG. 19, is configured of at least the lower screw extruder 98 having built therein an extruding screw 99 including a pressing screw portion 991 for pressing the ceramic material 80 toward a shaping die 91 and a dispersing screw portion 992 located on the same axis as the pressing screw portion 991 at the forward end thereof. The dispersing screw portion 992, as shown in FIG. 20, for example, can improve the uniformity of the ceramic material 80 by dispersing the uneven portion 89 thereof in a comma pattern or a swirl pattern (See Japanese Unexamined Patent Publication No. 2002-234012, for example).
The conventional extrusion molding apparatus described above, however, has yet to solve the problem described below.
Specifically, the mere dispersion of the ceramic material by the dispersing screw cannot attain a uniform density or a uniform fluidity in some cases. Especially in extrusion molding of a ceramic molded product of a honeycombed structure having thin partitioning walls, this problem often presents itself conspicuously. Specifically, in the case where the density and fluidity of the ceramic material are uneven, different partitioning walls develop different molding rates. The distortion caused by different molding rates is highly liable to deform the ceramic molded product as a whole.